Under what conditions do nonviolent demonstrations escalate to violence? I answer this question using a novel theory of individual preference formation in protests that begin peacefully. Rather than considering protest groups as unitary actors, I present a theory of collective action in which a group's decision is the product of a probabilistic aggregation of individual preferences. I argue that individuals involved in a nonviolent demonstration use the immediacy of their needs and the sustainability of collective action to decide whether or not to initiate violence against the state. Specifically, I hypothesize that the likelihood of violent escalation will increase when the relative food price is high, a demonstration is rural, and the event is spontaneous. An analysis of nonviolent demonstrations in Africa and Latin America largely supports my expectations.